A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The positioning devices that are used in conventional lithographic apparatus to provide forces to move the patterning device support (e.g. mask table) MT and the substrate table WT usually include a plurality of actuators. The actuators include copper coils attached to one part of the apparatus and a magnet assembly attached to the other part of the apparatus. When a current is passed through such coil, the interaction of the current passing through the coil and the magnetic field generated by the magnet produces a force between two parts of the apparatus. The coils of conventional actuators are formed from insulated wire that is wound in an orthocyclic fashion. Such coil is formed from conductive wire, for instance copper wire wound about a winding axis. To prevent short circuits between respective turns of the copper wire, the wire is encased in an electrically insulating material.
With conventional coil designs made up of orthocyclically wound wire, the heat transmission through the coil is low. Each insulated piece of wire is only in line-contact with the adjacent pieces of wire, limiting the area across which heat may be conducted. Furthermore, the material used to electrically insulate the wires from one another tends to be a poor conductor of heat further reducing the heat transfer characteristic across the coil as a whole. Consequently, a significant portion of the heat generated on the inner side of the coil is dissipated to the environment surrounding the coil, rather than being transferred through the coil to cooling elements arranged close to the coil.
To avoid these drawbacks, U.S. Pat. No. 6,891,600, the contents of which is herein incorporated in its entirety by reference, proposes to provide in a positioning device of a lithographic apparatus a planar motor having a stator and a translator, one of the stator and the translator including a periodic magnet structure and the other of the stator and the translator including a plurality of coils that can carry an electric current, the coils include a strip of electrically conductive sheet-material. This provides a coil with improved heat transfer characteristics since heat is transferred across the width of the strips of electrically conductive sheet-material. This is beneficial because the electrically conductive material has a higher thermal conductivity than the insulating material used in conventional coil designs. Due to the use of a wound strip of sheet material a better heat transfer is obtained.
However, a drawback of coils made of a strip of sheet material is that they have relative sharp edges. These sharp edges in combination with a small distance of the coils with respect to other objects, for instance cooling plates, result in high electric field intensity near the edges of the coils. This is a critical volume for partial discharge initialization and more generally high field strength related breakdown and functional lifetime threatening effects.